Important characteristics of the electro-optic response of conventional nematic liquid crystal devices and displays are the switching rise τrise and fall τfall times, which usually are in the range of a couple of milliseconds. It is well known that τrise can effectively be controlled by the applied electric field whereas τfall cannot. In many device applications of the liquid crystals, such as 3D LCDs for instance, fast switching of the liquid crystal device with both τrise and τfall being preferably in the microsecond region, is required. To achieve such a fast switching in nematic liquid crystal devices and displays is however a difficult and very complicated task especially what concerns τfall.
As known, τfall depends strongly on the liquid crystal material properties, elastic constants and viscosity. It depends also on the characteristics of the sandwich cell containing the liquid crystal, cell gap d and anchoring strength W. In the presence of strong anchoring condition, τfall is proportional to the square of the cell gap, i.e. d2, whereas if the anchoring condition is weak then τfall is proportional to d and 1/W. To decrease the cell gap is obviously a possible way of reducing τfall. Such an approach, however, is not easy to employ due to the limitations of the LCD technology.
Another possible way of reducing τfall is to increase the contact area between the liquid crystal and the solid surface.
One known way of achieving this is to create a polymer network within the liquid crystal bulk. Thus, the effect of the restoring force of the liquid crystal/solid surface interactions on the relaxation process, taking place in the liquid crystal after turning off the applied electric field, is substantially magnified resulting in reduction of τfall.
This approach is described in US 2010/0245723, which discloses a liquid crystal device of the twisted nematic type where a polymer network is disposed among the liquid crystal molecules of the liquid crystal layer. The polymer network is arranged to bias the liquid crystal molecules towards an untwisted state.
This will reduce the relaxation time (from the twisted state to the untwisted state), which is expected to result in a reduction of the total response time (τrise+τfall) of the liquid crystal device according to US 2010/0245723.
However, the polymer network in the liquid crystal device according to US 2010/0245723 will at the same time increase the rotation time (the time to transition the liquid crystal material from the untwisted state to the twisted state). Furthermore, it is likely that the introduction of the polymer network will result in an increase in the threshold voltage for switching as compared to the case without the polymer network. Further undesirable effects that may be introduced through the provision of a polymer network such as that in the liquid crystal device according to US 2010/0245723 include light scattering and residual birefringence, which may detrimentally influence the optical performance of the liquid crystal device.